I. The founder of the Marcosians (q.v.) and elder contemporary of St. Irenaeus, who, c. A.D. 175, in his refutation addresses him as one apparently still living (Adv. Hair., I, xi, 3, where the "clarus magister" is Marcus, not Epiphanes; and I, xiii, 21). Irenaeus, from whom St. Epiphanius (Haer., xxxiv) and St. Hippolytus (Haer., VI, xxxix-lv) quote, makes Marcus a disciple of Valentinus (q.v.), with whom Marcus's aeonology mainly agrees. St. Jerome (Ep. 75, 3) makes him a follower of Basilides (q.v.), confusing him no doubt with Marcus of Memphis. Clement of Alexandria, himself infected with Gnosticism, actually uses Marcus's number system though without acknowledgment (Strom., VI, xvi). Marcus first taught in Asia Minor and possibly later in the West also. His immoralities and juggling tricks (coloring the contents of the cup and increasing the quantity) are described by Irenaeus and Hippolytus. (For his system see Marcosians.)

II. One of the two defenders of Marcionism in Adamantius's Dialogue "De Recta in Deum fide"; the other is called Megethius; but whether these are fictitious or real personages is uncertain. Marcus's dualism is more absolute than that of Marcion himself: the demiurgus is the absolute evil principle. He inclines further towards Apelles, accepting salvation neither for the body nor the psyche but only for the pneuma.

A Manichean Gnostic, a native of Memphis, who introduced dualistic doctrines into Spain about the middle of the fourth century. His precise activity was unknown even to Sulpicius Severus (Hist. Sacr., II, xliv), c. A.D. 400, who only knows that he had two hearers or disciples: Agape, a wealthy matron, and the orator Elpidius, who became the instructors of Priscillian ("ab his Priscillianus est institutus") when still a layman. Elpidius and Priscillian were both condemned by the Council of Saragossa, but Elpidius did not share Priscillian's tragic fate in A.D. 385.